The present invention relates to a contact lens having improved dimensional stability, a contact lens blank having improved dimensional stability and, more particularly, relates to a spheric contact lens, a toric contact lens, a multifocal contact lens, and a contact lens blank constructed from a copolymer of 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate, and having either an absolute water balance ratio greater than 8 or a relative water balance ratio greater than 2 relative to a polymer of 2-hydroxyethyl methacrylate.
The prior art describes many polymers, based on acrylates or methacrylates, for use in contact lenses. For instance, U.S. Pat. No. 4,056,496 to Mancini et al. discloses a hydrogel formed by bulk polymerization of a dihydroxyalkyl acrylate or methacrylate, such as GMA; an alkyl acrylate or methacrylate; and a minor amount of an epoxidized alkyl acrylate or methacrylate. Additionally, U.S. Pat. No. 3,985,697 to Urbach teaches terpolymer hydrogels formed from hydroxyalkyl acrylate or hydroxyalkyl methacrylate, a non-water-soluble acrylate or methacrylate diester as a cross-linking agent, and an alkenoic carboxylic acid, such as acrylic or methacrylic acid. U.S. Pat. No. 3,947,401 to Stamberger discloses a bulk-polymerized water-insoluble, but water-swellable, copolymer formed from a polymerizable monoester of acrylic or methacrylic acid, such as 2-hydroxyethyl methacrylate (HEMA), and glycidyl methacrylate. Macret et al. in Polymer, 23(5), 748-753 (1982) discloses hydrogels prepared by radical polymerization of 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate, but this document neither discloses nor suggests a contact lens having improved dimensional stability or superior water balance.
Conventional non-ionic hydrogels constructed from methyl methacrylate (MMA) copolymers derive their strength from the methacrylate polymer backbone, but depend upon the pendant hydrophilic groups of the comonomers for water content. An exemplary hydrophilic comonomer is N-vinylpyrrolidone (NVP). The structure and amount of these hydrophilic components are limited by their compatibility with the hydrophobic MMA.
HEMA-based hydrogels have a hydrophilic core that permits a water content of 38%. Higher water contents are achieved by inclusion of either methacrylic acid (MAA) comonomer in ionic hydrogels, or hydrophilic comonomers in non-ionic systems. NVP has been a key monomer in attaining water contents up to 70%, but use of this comonomer results in temperature sensitivity during manufacturing. Moreover, progressive yellowing with age and changes in optical parameters as a result of temperature-dependent dimensional changes have also been observed with lenses constructed from these compositions.
The ability of a hydrogel lens to maintain its water-saturated state is essential for maximum lens stability. All hydrogel lenses dehydrate, for water evaporates continuously from the surface of a hydrogel lens. Dehydration of a contact lens results in a change in the dimensions of the lens, hence dehydration has a direct effect upon dimensional stability. Conventional contact lenses undergo a significant degree of dehydration during use and, accordingly, have a significant degree of dimensional instability, particularly at higher water contents.
Further, rehydration is important to the dimensional stability of a contact lens. If a lens material can be constructed which absorbs water more rapidly, then the lens will more closely return to a water-saturated state during each blink, when the lens is bathed in tear fluid. Therefore, as a lens begins to dehydrate, a characteristic of rapid rehydration is extremely advantageous for maintaining saturation and maximum stability. Unfortunately, conventional contact lens development either has ignored the effect of rehydration rate upon lenses or has constructed lenses of materials with a less than optimal rate of rehydration.
As such, there remains a need for a contact lens possessing superior dimensional stability and having a low rate of dehydration coupled with a high rate of rehydration.